The cytoskeleton is a network of fibers composed of proteins contained within a cell's cytoplasm. Depending on the definition used, the term "cytoskeleton" may refer only to the filamentous structures themselves, which are composed of a small set of proteins, or may include the hundreds of associated proteins like molecular motors, crosslinkers, and nucleation promoting factors. These may completely change the appearance, dynamic behaviour and functions of the cytoskeleton, despite similar composition. Although the name implies it to be stable, the cytoskeleton is mostly a highly dynamic structure, at least parts of which are constantly destroyed, renewed or newly constructed. Depending on cell type, the cytoskeleton can have multiple functions: It gives the cell shape and mechanical resistance to deformation; it can actively contract, thereby deforming the cell and the cell's environment and allowing cells to migrate; it is involved in many cell signaling pathways; it segregates chromosomes during cellular division; it provides a scaffold to organize the contents of the cell in space  and for intracellular transport (for example, the movement of vesicles and organelles within the cell); it can be a template for the construction of a cell wall; furthermore it forms specialized structures such as flagella, cilia, lamellipodia and podosomes. Originally thought to be unique to eukaryotes, recent research has identified the prokaryotic cytoskeleton.
Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments which are composed of actin, intermediate filaments which have around 70 different proteins as building blocks, and microtubules with tubulin as the basic subunit. The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol, it adds to the level of macromolecular crowding in this compartment. Cytoskeletal elements interact extensively and intimately with cellular membranes. A number of small molecule cytoskeletal drugs have been discovered that interact with actin and microtubules. These compounds have proven useful in studying the cytoskeleton and several have clinical applications.
Microfilaments (actin filaments)
Microfilaments are the thinnest filaments of the cytoskeleton. They are composed of linear polymers of G-actin subunits, and generate force when the growing (plus) end of the filament pushes against a barrier, such as the cell membrane. They also act as tracks for the movement of myosin molecules that attach to the microfilament and "walk" along them. Myosin motoring along F-actin filaments generates contractile forces in so-called actomyosin fibers, both in muscle as well as most non-muscle cell types. Actin structures are controlled by the Rho family of small GTP-binding proteins such as Rho itself for contractile acto-myosin filaments ("stress fibers"), Rac for lamellipodia and Cdc42 for filopodia.
These filaments, averaging 10 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton. Like actin filaments, they function in the maintenance of cell-shape by bearing tension (microtubules, by contrast, resist compression but can also bear tension during mitosis and during the positioning of the centrosome). Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles and serving as structural components of the nuclear lamina. They also participate in some cell-cell and cell-matrix junctions.
Different intermediate filaments are:
- made of vimentins. Vimentin intermediate filaments are in general present in mesenchymal cells.
- made of keratin. Keratin is present in general in epithelial cells.
- neurofilaments of neural cells.
- made of lamin, giving structural support to the nuclear envelope.
Microtubules are hollow cylinders about 23 nm in diameter (lumen = approximately 15 nm in diameter), most commonly comprising 13 protofilaments that, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behavior, binding GTP for polymerization. They are commonly organized by the centrosome.
In nine triplet sets (star-shaped), they form the centrioles, and in nine doublets oriented about two additional microtubules (wheel-shaped), they form cilia and flagella. The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton.
They play key roles in:
- intracellular transport (associated with dyneins and kinesins, they transport organelles like mitochondria or vesicles).
- the axoneme of cilia and flagella.
- the mitotic spindle.
- synthesis of the cell wall in plants.
|Cytoskeleton type||Diameter (nm)||Structure||Subunit examples|
|Intermediate filaments||10||two anti-parallel helices/dimers, forming tetramers|
|Microtubules||23||protofilaments, in turn consisting of tubulin subunits in complex with stathmin||α- and β-tubulin|
The cytoskeleton was once thought to be a feature only of eukaryotic cells, but homologues to all the major proteins of the eukaryotic cytoskeleton have been found in prokaryotes. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous. However, some structures in the bacterial cytoskeleton may have yet to be identified.
FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of guanosine triphosphate (GTP), but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new cell wall between the dividing cells.
MreB and ParM
Prokaryotic actin-like proteins, such as MreB, are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis.
Some plasmids encode a partitioning system that involves an actin-like protein ParM. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis.
The bacterium Caulobacter crescentus contains a third protein, crescentin, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and vibrioid forms of bacteria, but the mechanism by which it does this is currently unclear.
In 1903, Nikolai K Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton. The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929  while the term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert in 1931.
A fourth eukaryotic cytoskeletal element, microtrabeculae, was proposed by Keith Porter based on images obtained from high-voltage electron microscopy of whole cells in the 1970s. The images showed short, filamentous structures of unknown molecular composition associated with known cytoplasmic structures. Porter proposed that this microtrabecular structure represented a novel filamentous network distinct from microtubules, filamentous actin, or intermediate filaments. It is now generally accepted that microtrabeculae are nothing more than an artifact of certain types of fixation treatment, although the complexity of the cell's cytoskeleton is not yet fully understood.
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|Wikimedia Commons has media related to Cytoskeleton.|
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- Animation of leukocyte adhesion (Animation with some images of actin and microtubule assembly and dynamics.)
- http://cellix.imba.oeaw.ac.at/ Cytoskeleton and cell motility including videos
- Open access review article on the emergent complexity of the cytoskeleton (appeared in Advances in Physics, 2013)